The present invention relates to a method for the vapor phase carbonylation of alkyl alcohols, ethers and ester-alcohol mixtures to produce esters and carboxylic acids, and particularly the carbonylation of methanol to produce acetic acid and methyl acetate. More particularly, the present invention relates to a vapor phase carbonylation using a supported catalyst which includes a catalytically effective amount of iridium and gold.
Lower carboxylic acids and esters such as acetic acid and methyl acetate have been known as industrial chemicals for many years. Acetic acid is used in the manufacture of a variety of intermediary and end-products. For example, an important derivative is vinyl acetate which can be used as monomer or co-monomer for a variety of polymers. Acetic acid itself is used as a solvent in the production of terephthalic acid, which is widely used in the container industry, and particularly in the formation of PET beverage containers.
There has been considerable research activity in the use of metal catalysts for the carbonylation of lower alkyl alcohols, such as methanol, and ethers to their corresponding carboxylic acids and esters, as illustrated in equations 1-3 below:
ROH+COxe2x86x92RCOOHxe2x80x83xe2x80x83(1)
2ROH+COxe2x86x92RCOOR+waterxe2x80x83xe2x80x83(2)
ROR+COxe2x86x92RCOORxe2x80x83xe2x80x83(3)
Carbonylation of methanol is a well known reaction and is typically carried out in the liquid phase with a catalyst. A thorough review of these commercial processes and other approaches to accomplishing the formation of acetyl from a single carbon source is described by Howard et al. in Catalysis Today, 18 (1993) 325-354.
Generally, the liquid phase carbonylation reaction for the preparation of acetic acid using methanol is performed using homogeneous catalyst systems comprising a Group VIII metal and iodine or an iodine-containing compound such as hydrogen iodide and/or methyl iodide. Rhodium is the most common Group VIII metal catalyst and methyl iodide is the most common promoter. These reactions are conducted in the presence of water to prevent precipitation of the catalyst. However, solid heterogeneous carbonylation catalysts offer the potential advantages of easier product separation, lower cost materials of construction, facile recycle, and even higher rates. The use of solid carbonylation catalyst in a vapor phase carbonylation reaction is especially beneficial due to the fact that operating in the vapor phase eliminates catalyst dissolution, i.e., metal leaching from the catalyst support, which occurs in the known heterogeneous processes operating in the presence of liquid compounds.
Rhodium was the first heterogeneous catalyst used in vapor phase carbonylation. Schultz, in U.S. Pat. No. 3,689,533, discloses using a supported rhodium heterogeneous catalyst for the carbonylation of alcohols to form carboxylic acids in a vapor phase reaction. Schultz further discloses the presence of a halide promoter. Schultz in U.S. Pat. No. 3,717,670 goes further to describe a similar supported rhodium catalyst in combination with promoters selected from Groups IB, IIIB, IVB, VB, VIB, VIII, lanthanide and actinide elements of the Periodic Table. Schultz teaches that these elements are useful to promote the rhodium activity, but do not themselves provide carbonylation catalysis. Uhm, in U.S. Pat. No. 5,488,143, describes the use of the alkali metals Li, Na, K, Rb, and Cs, the alkaline earth metals Be, Mg, Ca, Sr, and Ba, or the transition metals Co, Ru, Pd, Pt, Os, Ir, N-i, Mn, Re, Cr, Mo, W, V, Nb, Ta, Ti, Zr, and Hr as promoters for supported rhodium for the halide-promoted, vapor phase methanol carbonylation reaction. Further, Pimblett, in U.S. Pat. No. 5,258,549, teaches that the combination of rhodium and nickel on a carbon support is more active than either metal by itself.
Iridium is also an active catalyst for methanol carbonylation reactions but normally provides reaction rates lower than those offered by rhodium catalysts when used under otherwise similar conditions.
U.S. Pat. No. 5,510,524 teaches that the addition of rhenium improves the rate and stability of both the Ir-I and Rh-I homogeneous catalyst systems.
European Patent Application EP 0 752 406 A1 teaches that ruthenium, osmium, rhenium, zinc, cadmium, mercury, gallium, indium, or tungsten improve the rate and stability of the liquid phase Ir-I catalyst system. Generally, the homogeneous carbonylation processes presently being used to prepare acetic acid provide relatively high production rates and selectivity. However, heterogeneous catalysts offer the potential advantages of easier product separation, lower cost materials of construction, facile recycle, and even higher rates.
EP 0 759 419 A1 discloses a carbonylation process comprising a first carbonylation reactor wherein an alcohol is carbonylated in the liquid phase in the presence of a homogeneous catalyst system and the off gas from this first reactor is then mixed with additional alcohol and fed to a second reactor containing a supported catalyst. The homogeneous catalyst system utilized in the first reactor comprises a halogen component and a Group VIII metal selected from rhodium and iridium. When the Group VIII metal is iridium, the homogeneous catalyst system also may contain an optional co-promoter selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, indium and gallium. The supported catalyst employed in the second reactor comprises a Group VIII metal selected from the group consisting of iridium, rhodium, and nickel, and an optional metal promoter on a carbon support. The optional metal promoter may be iron, nickel, lithium and cobalt. The conditions within the second carbonylation reactor zone are such that mixed vapor and liquid phases are present in the second reactor. The presence of a liquid phase component in the second reactor inevitably leads to leaching of the active metals from the supported catalyst which, in turn, results in a substantial decrease in the activity of the catalyst.
In addition to the use of iridium as a homogeneous alcohol carbonylation catalyst, Paulik et al., in U.S. Pat. No. 3,772,380, describe the use of iridium on an inert support as a catalyst in the vapor phase, halogen-promoted, heterogeneous alcohol carbonylation process.
Evans et al., in U.S. Pat. No. 5,185,462, describe heterogeneous catalysts for halide-promoted vapor phase methanol carbonylation based on noble metals attached to nitrogen or phosphorus ligands attached to an oxide support.
Nickel on activated carbon has been studied as a heterogeneous catalyst for the halide-promoted vapor phase carbonylation of methanol, and increased rates are observed when hydrogen is added to the feed mixture. Relevant references to the nickel-on-carbon catalyst systems are provided by Fujimoto et al. in Chemistry Letters (1987) 895-898 and in Journal of Catalysis, 133 (1992) 370-382 and in the references contained therein. Liu et al., in Ind. Eng. Chem. Res., 33 (1994) 488-492, report that tin enhances the activity of the nickel-on-carbon catalyst. Mueller et al., in U.S. Pat. No. 4,918,218, disclose the addition of palladium and optionally copper to supported nickel catalysts for the halide-promoted carbonylation of methanol. In general, the rates of reaction provided by nickel-based catalysts are lower than those provided by the analogous rhodium-based catalysts when operated under similar conditions.
Other single metals supported on carbon have been reported by Fujimoto et al. in Catalysis Letters, 2 (1989) 145-148 to have limited activity in the halide-promoted vapor phase carbonylation of methanol. The most active of these metals is Sn. Following Sn in order of decreasing activity are Pb, Mn, Mo, Cu, Cd, Cr, Re, V, Se, W, Ge and Ga. None of these other single metal catalysts are nearly as active as those based on Rh, Ir, or Ni.
U.S. Pat. No. 5,218,140, to Wegman, describes a vapor phase process for converting alcohols and ethers to carboxylic acids and esters by the carbonylation of alcohols and ethers with carbon monoxide in the presence of a metal ion exchanged heteropoly acid supported on an inert support. The catalyst used in the reaction includes a polyoxometallate anion in which the metal is at least one of a Group V(a) and VI(a) is complexed with at least one Group VIII cation such as Fe, Ru, Os, Co, Rh, Ir, Ni, Pd or Pt as catalysts for the halide-free carbonylation of alcohols and other compounds in the vapor phase. The process does not utilize a halide cocatalyst.
Various solid support materials have been reported as useful in halide-promoted heterogeneous vapor phase carbonylation systems. European Patent Applications EP 0 120 631 A1 and EP 0 461 802 A2 describe the use of special carbons as supports for carbonylation catalysts having a single transition metal component chosen from Co, Ru, Fe, Ni, Rh, Pd, Os, Ir, Pt, and Group VIII metals. The literature contains several reports of the use of rhodium-containing zeolites as vapor phase alcohol carbonylation catalysts at one bar pressure in the presence of halide promoters. The lead references on this type of catalyst are presented by Maneck et al. in Catalysis Today, 3 (1988), 421-429. Gelin et al., in Pure and Appli Chem., Vol 60, No. 8, (1988) 1315-1320, provide examples of the use of rhodium or iridium contained in zeolite as catalysts for the vapor phase carbonylation of methanol in the presence of halide promoter. Krzywicki et al., in Journal of Molecular Catalysis, 6 (1979) 431-440, describe the use of silica, alumina, silica-alumina and titanium dioxide as supports for rhodium in the halide-promoted vapor phase carbonylation of methanol. Luft et al., in U.S. Pat. No. 4,776,987 and in related disclosures, describe the use of chelating ligands rchemically attached to various supports as a means to attach Group VIII metals to a heterogeneous catalyst for the halide-promoted vapor phase carbonylation of ethers or esters to carboxylic anhydrides. Drago et al., in U.S. Pat. No. 4,417,077, describe the use of anion exchange resins bonded to anionic forms of a single transition metal as catalysts for a number of carbonylation reactions including the halide-promoted carbonylation of methanol.
A number of solid materials have been reported to catalyze the carbonylation of methanol without the addition of the halide promoter. Gates et al., in Journal of Molecular Catalysis, 3 (1977/78) 1-9, describe a catalyst containing rhodium attached to polymer bound polychlorinated thiophenol for the liquid phase carbonylation of methanol.
Smith et al., in European Patent Application EP 0 596 632 A1, describe the use of mordenite zeolite containing Cu, Ni, Ir, Rh, or Co as catalysts for the halide-free carbonylation of alcohols. Feitler, in U.S. Pat. No. 4,612,387, describes the use of certain zeolites containing no transition metals as catalysts for the halide-free carbonylation of alcohols and other compounds in the vapor phase.
Certain disadvantages present in the prior art include instability of the carbonylation catalysts, lack of product selectivity, and poor product separation. Accordingly, there is a need for an alternative catalyst which can be used in a vapor phase carbonylation process for the production of carboxylic acids and their esters and in which the catalyst is maintained in the solid phase.
Briefly, the present invention is a heterogeneous vapor phase carbonylation process wherein an iridium-gold solid supported catalyst is used. The process includes feeding a gaseous mixture of reactants comprising lower alkyl alcohols, ethers and ester-alcohol mixtures and carbon monoxide to a carbonylation zone containing a solid supported catalyst comprising a catalytically effective amount of iridium and gold associated with a solid support material that, desirably, is inert to the carbonylation reaction.
Another aspect of the invention relates to a carbonylation catalyst for producing esters and carboxylic acids in a vapor phase carbonylation process having a solid supported catalyst component and further includes a halogen and/or halide containing compound, (collectively referred to herein as a xe2x80x9chalidexe2x80x9d).
It is an object of the present invention to provide a solid phase catalyst composition for vapor phase carbonylation of methanol to form acetic acid or methyl acetate.
It is another object of the invention is to provide a carbonylation method that results in higher yields of acetic acid with minimum formation of ethers, aldehydes, and other undesirable by-products.
These and other objects and advantages of the invention will become apparent to those skilled in the art from the accompanying detailed description.
The solid supported catalyst used in the present vapor phase carbonylation process includes a catalytically effective amount of iridium and gold associated with a solid support material. The solid supported catalyst of the present invention is particularly useful in the continuous production of carboxylic acids and esters by reacting lower alkyl alcohols, polyols, ethers, esters or a mixture thereof with carbon monoxide during carbonylation, especially vapor-phase carbonylation. The vapor phase carbonylation method of the present invention is particularly useful for the continuous production of acetic acid, methyl acetate and mixtures thereof.
It was surprising to find that gold is an effective cocatalyst with iridium in carbonylation since gold has not heretofore been described as having or contributing to carbonylation activity. It was especially surprising to find that the combination of iridium and gold is a much better catalyst than the summation of their reactivities when used separately as carbonylation catalysts. The rate of vapor phase carbonylation of the present iridium-gold catalyzed process was surprisingly found to be from about 30 to about 45 percent higher than the summation of the reaction rates of an iridium catalyzed reaction and a gold catalyzed reaction using the same concentration of corresponding metals.
The carbonylation process of the present invention comprises feeding a gaseous mixture of an alkyl alcohol, ether, ester, or mixture thereof and carbon monoxide to a carbonylation zone and recovering a gaseous carboxylic acid, ester, or mixture product. The carbonylation zone is maintained under vapor-phase carbonylation conditions of temperature and pressure and contains a supported catalyst comprising a catalytically effective amount of iridium and gold associated with a solid support material.
In the catalyst of the present invention, a catalytically effective amount of iridium and gold are associated with a solid support material that is inert in a carbonylation reaction environment. For clarification of terms used herein, the term xe2x80x9ccatalytically effectivexe2x80x9d is used herein to refer to catalysis of the carbonylation of a carbonylatable compound. Further, iridium and gold atoms are xe2x80x9cassociatedxe2x80x9d with the solid support material when the iridium and gold atoms are disposed on, through, and/or near the solid support as a result of any type of chemical and/or physical relationship.
A material suitable for use as the solid catalyst support material in the present invention is a porous solid having a size of from about 400 mesh per inch to about 0.5 mesh per inch. The shape of the solid support is not particularly important and can be regular or irregular and include extrudates, rods, balls, broken pieces and the like disposed within the reactor.
The support is preferably carbon, or activated carbon, having a high surface area. Activated carbon is well known in the art and may be derived from a variety of sources including coal, peat, and coconut shells having a density of from about 0.03 grams/cubic centimeter (g/cm3) to about 2.25 g/cm3. The carbon can have a surface area of from about 200 square meters/gram (m2/g) to about 1200 m2/g. Other solid support materials may be used, either alone or in combination, in accordance with the present invention include pumice, alumina, silica, silica-alumina, magnesia, diatomaceous earth, bauxite, titania, zirconia, clays, magnesium silicate, silicon carbide, zeolites, ceramics, and combinations thereof.
The compound or form of iridium used to prepare the catalyst is not critical, and the catalyst may be prepared from any of a wide variety of iridium containing compounds. Indeed, iridium compounds containing myriad combinations of halide, trivalent nitrogen, organic compounds of trivalent phosphorous, carbon monoxide, hydrogen, and 2,4-pentane-dione, either alone or in combination. Such materials are available commercially and may be used in the preparation of the catalysts utilized in the present invention. In addition, the oxides of iridium may be used if dissolved in the appropriate medium. The iridium used in this invention is preferably an iridium chloride, such as iridium trichloride or hydrated trichloride, hexacholoro-iridate and any of the various salts of hexachloro-iridate (IV).
Similarly, the compound or form of gold used to prepare the catalyst generally is not critical, and may be selected from any of a variety of compounds containing gold, their respective salts, and mixtures thereof. Particularly useful gold compounds include gold halides, cyanides, hydroxides, oxides, sulfides, and phosphine complexes either alone or in combination. Such materials are available commercially and may be used in the preparation of the catalysts used in the process of the present invention. Gold oxide may be used if dissolved in the appropriate medium. However, the compound used to provide the gold component is preferably in a water soluble form. Preferred water soluble gold sources include halides, particularly the tetrahaloaurates. The most preferred hydrogen tetrahaloaurates are hydrogen tetrachloroaurate (III) and hydrogen tetrabromoaurate (III).
The amount of iridium and gold on the support can vary from about 0.01 weight percent to about 10 weight percent, with from about 0.1 weight percent to about 2 weight percent of each component being preferred. The weight percent of each said metal is determined as the weight of atoms of that particular metal compared to the total weight of the solid supported catalyst composition.
Further, the molar ratio of iridium to gold is preferably in a range from about 0.1:1 to about 10:1, with a molar ratio of about 0.5:1 to about 3:1 iridium to gold being more preferred.
The catalyst of the present invention is very effective in carbonylation when there are essentially no other metals associated with the support besides iridium and gold. However, other metals may be associated with the support as part of the catalyst composition, either as promoters, as co-catalysts, or as inert metals, as long as the amount of iridium and gold present is a sufficient amount so that the iridium and gold effectively catalyze carbonylation in the presence of the other associated metal. If other such metals are associated with the solid support, the ratio of the weight of gold to the weight of the metals other than iridium and gold is preferably greater than 1:1, with a ratio of at least about 2:1 being more preferable. Suitable metals for association with the support besides iridium and gold are most likely alkaline or alkaline earth metals, tin, vanadium, molybdenum, and tungsten.
The present solid supported catalyst may be prepared by depositing iridium and gold on the solid support material to form a composition wherein a catalytically effective amount of iridium and gold are associated with the solid support material. The iridium and gold may be deposited concurrently or separately. The deposition of iridium and gold may be conducted by any means sufficient to cause the iridium and gold to associate with the support including but not limited to methods employing heat, electrolyzing, physical embedding, sonification, impregnating, co-precipitation. The preferred method of depositing the iridium and gold on the support is by dissolving or dispersing iridium and gold compounds in an appropriate solvent, either in one solution together or in two separate solutions, and contacting, preferably impregnating, the support with the iridium and gold containing solutions to provide a wet solid support material. The iridium and gold atoms are then associated with the support when the solvent is removed by drying the wet support material.
Various methods of contacting the support material with the iridium and gold may be employed as long as the contacting method provides association between the iridium and gold atoms and the support. For example, an iridium containing solution can be admixed with a gold solution prior to impregnating the support material. Alternatively, the respective solutions can be impregnated separately into or associated with the support material prior to impregnating the support material with the second solution. For example, the gold component may be deposited on a previously prepared catalyst support having the iridium component already incorporated thereon. Desirably, in this alternative embodiment, the support is dried prior to contacting the second solution. Similarly, the iridium and gold may be associated with the support material in a variety of forms. For example, slurries of the iridium and gold can be poured over the support material. Alternatively, the support material may be immersed in excess solutions of the active components with the excess being subsequently removed using techniques known to those skilled in the art. The solvent or liquid is evaporated, i.e. the solid support is dried so that at least a portion of the iridium and gold is associated with the solid support. Drying temperatures can range from about 100xc2x0 C. to about 600xc2x0 C. One skilled in the art will understand that the drying time is dependent upon the temperature, humidity, and solvent. Generally, lower temperatures require longer heating periods to effectively evaporate the solvent from the solid support.
The liquid used to deliver the iridium and gold in the form of a solution, dispersion, or suspension is a liquid having a low boiling point, i.e., high vapor pressure at a temperature of from about 10xc2x0 C. to about 140xc2x0 C. Examples of suitable solvents include carbon tetrachloride, benzene, acetone, methanol, ethanol, isopropanol, isobutanol, pentane, hexane, cyclohexane, heptane, toluene, pyridine, diethylamine, acetaldehyde, acetic acid, tetrahydrofuran and water.
In the preferred embodiment of the present invention, the carbonylation catalyst further includes a halide promoter. The term xe2x80x9chalidexe2x80x9d is used generically and interchangeably with xe2x80x9chalogenxe2x80x9d, xe2x80x9chalidexe2x80x9d or xe2x80x9chalide containing compoundxe2x80x9d and includes both the singular or plural forms. It is preferable that the halide is promoter is present as a vapor. However, the halide may also be present as a liquid or as a solid, as long as the halide component is in sufficient contact with the iridium and gold components so as to provide iridium-halide and gold-halide complex formation. The halide promoter is a catalyst component instead of a reactant, in that it is essentially non-consumed in the present carbonylation process. The halide may be introduced at the catalyst preparation step or, preferably, is introduced into the carbonylation reactor with the gaseous reactants.
The halide promoter may include one or more of chlorine, bromine and/or iodine compounds and is preferably vaporous under vapor-phase carbonylation conditions of temperature and pressure. Suitable halides include hydrogen halides such as hydrogen iodide and gaseous hydriodic acid; alkyl and aryl halides having up to 12 carbon atoms such as, methyl iodide ethyl iodide, 1-iodopropane, 2-iodobutane, 1-iodobutane, methyl bromide, ethyl bromide, and benzyl iodide. Desirably, the halide is a hydrogen halide or an alkyl halide having up to 6 carbon atoms. Non-limiting examples of preferred halides are hydrogen iodide, methyl bromide and methyl iodide. The halide may also be a molecular halide such as I2, Br2, or Cl2.
The vapor phase carbonylation process of the present invention is conducted by contacting the vapor phase reactants with the catalyst by flowing them through or over the catalyst. This is accomplished by feeding a gaseous mixture comprising the reactants to a carbonylation zone containing the solid supported iridium-gold catalyst of the present invention. The present heterogeneous vapor-phase process preferably operates entirely in the gas phase, i.e., none of the compounds or materials present in the carbonylation zone or reactor exists in a mobile liquid phase. A gaseous product comprising a carboxylic acid, an ester thereof, or a mixture thereof are recovered from the carbonylation zone.
Vapor-phase carbonylation is typically operated at temperatures above the dew point of the product mixture, i.e., the temperature at which condensation occurs. However, since the dew point is a complex function of dilution, product composition and pressure, and particularly with respect to non-condensable gases such as unreacted carbon monoxide, hydrogen, or inert diluent gas, the process may still be operated over a wide range of temperatures, provided the temperature exceeds the dew point of the product effluent. In practice, this generally dictates a temperature range of about 100xc2x0 C. to 500xc2x0 C., with temperatures in the range of 100xc2x0 C. to 325xc2x0 C. being preferred and temperature of about 150xc2x0 C. to 275xc2x0 C. being particularly useful.
As with temperature, the useful pressure range is limited by the dew point of the product mixture. However, provided that the reaction is operated at a temperature sufficient to prevent liquefaction of the product effluent, a wide range of pressures may be used, e.g., pressures in the range of about 0.1 to 100 bars absolute. The process preferably is carried out at a pressure in the range of about 1 to 50 bars absolute, most preferably, about 3 to 30 bar absolute.
Suitable feedstocks for carbonylation using the present catalyst include lower alkyl alcohols, ethers, ester and esters-alcohol mixtures which may be carbonylated using the catalyst of the present invention. Non-limiting examples of feedstocks include alcohols and ethers in which an aliphatic carbon atom is directly bonded to an oxygen atom of either an alcoholic hydroxyl group in the compound or an ether oxygen in the compound and may further include aromatic moieties. Preferably, the feedstock is one or more lower alkyl alcohols having from 1 to 10 carbon atoms and preferably having from 1 to 6 carbon atoms, alkane polyols having 2 to 6 carbon atoms, alkyl alkylene polyethers having 3 to 20 carbon atoms and alkoxyalkanols having from 3 to 10 carbon atoms. The most preferred reactant is methanol. Although methanol is the preferred feedstock to use with the solid supported catalyst of the present invention and is normally fed as methanol, it can be supplied in the form of a combination of materials which generate methanol. Examples of such materials include (i) methyl acetate and water and (ii) dimethyl ether and water. During carbonylation, both methyl acetate and dimethyl ether are formed within the reactor and, unless methyl acetate is the desired product, they are recycled with water to the reactor where they are converted to acetic acid. Accordingly, one skilled in the art will further recognize that it is possible to utilize the catalyst of the present invention to produce a carboxylic acid from an ester feed material.
Although the presence of water in the gaseous feed mixture is not essential when using methanol, the presence of some water does serve to suppress formation of methyl acetate and/or dimethyl ether. Therefore, when using methanol to generate acetic acid, the molar ratio of water to methanol can be 0:1 to 10:1, but preferably is in the range of 0.01:1 to 1:1. When using an alternative source of methanol such as methyl acetate or dimethyl ether, the amount of water fed usually is increased to account for the mole of water required for hydrolysis of the methanol alternative. Accordingly, when using either methyl acetate or dimethyl ether, the mole ratio of water to ester or ether is in the range of 1:1 to 10:1, but preferably in the range of 1:1 to 3:1. In the preparation of acetic acid, it is apparent that combinations of methanol, methyl ester, and/or dimethyl ether are equivalent, provided the appropriate amount of water is added to hydrolyze the ether or ester to provide the methanol reactant.
When the present vapor-phase carbonylation process is used to produce methyl acetate, no water should be added and dimethyl ether becomes the preferred feedstock. Further, when methanol is used as the feedstock in the preparation of methyl acetate, it is necessary to remove water. However, the primary utility of the catalyst of the present invention is in the manufacture of acetic acid.
In practice, a gaseous mixture having at least one of lower alkyl alcohol, ether and ester-alcohol mixture, either alone or in combination; carbon monoxide; and a halide are fed to a carbonylation reactor containing the iridium and gold supported catalyst described above. The reactant, in the vapor phase, is allowed to contact the solid supported catalyst. The reactor is maintained under carbonylation conditions of temperature and pressure. If acetic acid is the desired product, the feedstock may consist of methyl alcohol, dimethyl ether, methyl acetate, a methyl halide or any combination thereof. If it is desired to increase the proportion of acid produced, the ester may be recycled to the reactor together with water or introduced into a separate reactor with water to produce the acid in a separate zone.
The carbon monoxide can be a purified carbon monoxide or include other gases. The carbon monoxide need not be of a high purity and may contain from about 1% by volume to about 99% by volume carbon monoxide, and preferably from about 70% by volume to about 99% by volume carbon monoxide. The remainder of the gas mixture may include such gases as nitrogen, hydrogen, carbon dioxide, water and paraffinic hydrocarbons having from one to four carbon atoms. Although hydrogen is not part of the reaction stoichiometry, hydrogen may be useful in maintaining optimal catalyst activity. The preferred ratio of carbon monoxide to hydrogen generally ranges from about 99:1 to about 2:1, but ranges with even higher hydrogen levels are also likely to be useful.
The amount of halide present in the gaseous feed to produce an effective carbonylation is based on the amount of alcohol or alcohol equivalents. The molar ratio of alcohol to halide ranges from about 1:1 to about 10,000:1, with the preferred range being from about 5:1 to about 1000:1.
In a preferred aspect of the invention, the vapor-phase carbonylation catalyst of the present invention may be used for making acetic acid, methyl acetate or a mixture thereof. The process includes the steps of contacting a gaseous mixture comprising methanol and carbon monoxide with the iridium-gold catalyst described above in a carbonylation zone and recovering a gaseous product from the carbonylation zone. The main gaseous products recovered include methyl acetate, acetic acid, unreacted methanol, and methyl iodide.